The present invention relates to a semiconductor integrated circuit and more particularly relates to a technique of switching transistors in a digital circuit at a high speed.
In a digital semiconductor integrated circuit, a transistor is equivalent to a switch that turns ON or OFF in response to a signal. To change the ON/OFF states of a transistor, it is necessary to release the charges that have been stored on a parasitic capacitance of the transistor or to supply charges to the transistor. However, it usually takes a rather long time for those charges to reach their destinations because various obstructions exist on the path of the charges. Among other things, a parasitic inductance associated with a power line very likely prevents charges from being supplied from a power supply to the transistor instantaneously. Accordingly, to reduce that parasitic inductance, an IC chip of today often includes bypass capacitors.
In applications handling a signal with a frequency on the order of several gigahertz, however, even those additional bypass capacitors cannot cope with the demand of storing charges in a transistor rapidly enough. That is to say, the charge supplying ability of a power supply system determines the frequency characteristic of an overall chip. More specifically, an instantaneous rise in current either decreases the supply voltage or increases the ground level unintentionally, thus limiting the amount of instantaneous current flowing.
It is therefore an object of the present invention to switch a transistor at a high speed by providing a circuit for pumping charges up or down compulsorily in the quantity required for turning the transistor ON or OFF.
To achieve this object, the present invention utilizes an active variable-capacitance device. Then, not just charges can be pumped up or down intentionally but also charges stored in a transistor are recyclable as well.
Specifically, an inventive semiconductor integrated circuit includes a switching device and a varactor device. The switching device has been formed on a semiconductor substrate to turn ON or OFF in response to a signal. The varactor device has a capacitance varying with a voltage level of the signal. The varactor device has been formed on the semiconductor substrate to help the switching device change its states at a high speed by exchanging charges with the switching device when the signal rises or falls.
To realize the charge exchange highly efficiently, the varactor device may be formed adjacently to the switching device. In one embodiment of the present invention, a variation in capacitance of the varactor device is preferably once to twice greater than a variation in capacitance of the switching device. In still another embodiment, the varactor device may be provided selectively either for a section that should realize high-speed signal transmission for the switching device or for an input/output circuit for the switching device.
The present invention utilizes a varactor device that can help a switching device change its states at a high speed by exchanging charges with the switching device when a signal rises or falls. Thus, the present invention realizes a semiconductor integrated circuit that can afford to handle even a signal with as high a frequency as 100 GHz.